where \(M_2=\frac{1}{3}zS(S+1)(3D/2)^2\)  is the second moment of the absorption line [1]} and \(z\)  = 6 denotes the number of nearest neighbours on the triangular lattice.
We stress that the estimated exchange anisotropy is stronger than the isotropic exchange interaction, \(D>J\)  (see main text), which justifies omission of the exchange-narrowing effects [1]}.
